FXYD proteins 1-7 are a family of single transmembrane-spanning proteins that are named after their shared FXYD amino acid motif in the extracellular domain. FXYD proteins are known for an effect they are believed to have on membrane proteins they associate with, in particular the membrane Na+—K+ ATPase. FXYD proteins do not form part of Na+—K+ pump per se but act as tissue specific modulators of Na+—K+ pump function.
FXYD proteins have been widely implicated in regulation of Na+—K+ pump function, and it is known that they indeed modify functional properties of the pump. In particular FXYD1, unique among FXYD proteins in that it can be phosphorylated on C-terminal serine residues, is understood to regulate the Na+—K+ pump in response to phosphorylation. However, a problem with this understanding is that FXYD proteins 2-7 have no functional phosphorylation sites and most tissues do not express FXYD1, thus phosphorylation of FXYD proteins cannot be a universally applicable scheme for Na+—K+ pump regulation by FXYD proteins.
The transmembrane gradients for Na+ and K+ maintained by the Na+—K+ pump are critical for generation of membrane potentials and hence cell excitability. They also drive secondary co- and counter-transport processes for other ions as well as for a large number of organic compounds, critical for cell metabolism. The pump and the wider superfamily of cation-transporting ATPases it belongs to are important therapeutic targets. However, despite having been discovered over 50 years ago modulation of Na+—K+ pump activity remains poorly understood.
The inventors have previously illustrated a role for oxidative signalling in regulation of the Na+—K+ pump in that the β1 subunit of the Na+—K+ pump α/β heterodimer has a “reactive” cysteine residue capable of being glutathionylated and that such glutathionylation reversibly inhibits Na+—K+ pump function. Glutathionylation can be induced by chemical oxidants or mediated by membrane receptor- and protein kinase-dependent activation of NADPH oxidase.
Oxidative stress is an important factor in a number of diseases including myocardial infarction, stroke and cancer. Increased oxidative stress and high levels of myocyte Na+ and Ca+ contribute to myocardial damage and contractile abnormalities in ischemia and reperfusion.
In response to oxidative stress, the β1 subunit of the Na+—K+ pump is glutathionylated in infarction and pump inhibition caused by this may contribute to the raised Na+ and Ca2+ levels. Oxidative stress has also been implicated in cerebral ischmaemic damage with stroke, and antioxidant strategies examined. Furthermore oxidative stress is known to be an important factor in ischemia reperfusion injury.
There remains a need for improved methods for the modulation of Na/K pump activity, such as for use in therapeutic context where inhibition of pump activity is associated with detriment to health.